The present invention relates to an achromatic phase shift device for introducing a wavelength independent optical phase shift in a first optical beam during operation, comprising at least one dispersive element. A second aspect of the present invention relates to an interferometer using such an achromatic phase shift device.
The American patent U.S. Pat. No. 5,862,001 describes a non-deviating prism with a continuously variable dispersion. This arrangement of optical elements allows to obtain a variable angular dispersion without any angular deviation at a central wavelength.
The most common means for obtaining a phase shift in an optical beam are phase shift devices using optical path delay means to influence the optical path of the beam and, thus, the phase of the optical beam. A disadvantage of such a known phase shift device is that the phase shift obtained is dependent on the wavelength. For applications in which the bandwidth of the light beam used is very small, this is not necessarily a problem. However, when a phase shift is needed in an optical beam with a broader bandwidth, the known device does not suffice.
This may for instance be the case for the application of an achromatic phase shift device in an interferometer, used in observation of planets near stars. These interferometers are for instance used in optical synthetic aperture systems, using multiple optical beams from different telescopes separated by a certain baseline. The optical beams from the telescopes usually have a broad wavelength. To be able to detect planets near stars, the light of the star is nulled in the interferometer by introducing a phase difference of xcfx80 radians between the interfering beams. Using known phase shift devices, a suppression factor of 100 may be obtained, while for certain detections a suppression factor of 106 is necessary.
In the prior art, achromatic phase shift in an optical beam is accomplished using an achromatic phase shift device, which comprises dispersive elements being formed by at least two plan parallel plates with a different refractive index. The dispersive elements effectuate a wavelength dependent optical path difference, and with the right combination of materials (refractive index) and dimensions of the plates, an achromatic phase shift can be accomplished over a certain wavelength range. However, the dimensions of the plates are fixed and, therefore, the phase shift accomplished is fixed. Also, at least two materials are needed with a different refractive index, which may be disadvantageous in numerous applications.
It is an object of the present invention to provide a phase shift device for producing a phase shift over a wide frequency range, i.e. an achromatic phase shift device. It is a secondary object of the present invention to provide an interferometer, which is particularly suited for planet detection near distant stars, by nulling the light from the associated star.
The first object is achieved by a phase shift device as disclosed herein.
The arrangement of the device allows introducing a phase shift in an optical beam travelling through the device, by varying the position of the first refractive means relative to the second refractive means. The first and second refractive means may be placed at a certain distance to one another and have small dimensions, which are sufficient for refracting the optical beam in the device as desired.
Using multiple pairs of respective first and second refractive means, it becomes possible to obtain a phase shift of an optical beam through the achromatic phase shift device, which is wavelength independent over a broad wavelength region.
In this embodiment, a perfect match of the predetermined phase shift is obtained for M+1 wavelengths.
In an embodiment of the achromatic phase shift device according to the present invention, the respective first refractive means of the multiple pairs are positioned adjacent to each other, forming a first group, the respective first refractive means in the first group being in physical contact. Preferably, also the respective second refractive means of the multiple pairs are positioned adjacent to each other, forming a second group, the respective second refractive means in the second group being in physical contact. By positioning the respective first and second refractive means such that they are in physical contact, interfaces occur between materials of different refractive index. The feature that the elements are in physical contact enables producing achromatic phase shift devices in a reliable and robust manner.
In a preferred embodiment of the achromatic phase shift device according to the present invention, the respective first and second refractive means of the multiple pairs are positioned symmetrically on respective sides of a first element pair. This allows a very compact arrangement of the device according to the present invention.
In a further embodiment, the refractive index of the first refractive means and the second refractive means of all of a specific pair of element pairs is substantially equal. This has advantages with respect to production of the device (only one dispersive material is needed), but also during operation, as environmental conditions will have less impact when all refractive means are made of the same material.
In certain arrangements of the first and second refractive means of the phase shift device, spaces may exist between the first refractive means and the second refractive means of each of the first pair and the plurality of further pairs. Preferably, these spaces are filled with a predetermined medium having a predetermined refractive index. The medium can be air, a liquid or vacuum. For further calculating purposes, these spaces can also be regarded as forming first and second refractive means.
To be able to use the device as a phase modulator, the device further comprises first control means for moving the first refractive means and the second refractive means of each of the first pair and the plurality of further pairs with respect to each other. Preferably, the direction of movement is perpendicular to a line of intersection of the input surface and output surface of the first refractive means. This allows variation of the distance travelled by the optical beam through the first and second refractive means and between the first and second refractive means of one or more of the first pair and the plurality of further pairs. This may be achieved very accurately by various control means known to the person skilled in the art.
It will be clear to the person skilled in the art that the change in position of the first refractive means relative to the second refractive means may be obtained by moving the first refractive means, the second refractive means, or both.
In an embodiment, the first and second refractive means are preferably formed by a first and a second prism, respectively. Such prisms with the required dimensions can be readily obtained or are easy to manufacture.
In a further embodiment, the device further comprises additional means of a dispersive material for applying a further chromatic correction to the optical beam, in which the dispersive material has a refractive index, which is different from the refractive index of the first and second refractive means. This embodiment enables a further achromatic correction, diminishing the higher order wavelength dependent errors in the chromatic correction.
In an alternative embodiment the sum of the first and second distance of the first and second refractive means and the required optical path are determined by requiring constant terms and terms with xcex2, xcex3 . . . xcexM to become zero and the term with xcex to become equal to "psgr"0/2xcfx80 in the equation for the introduced optical path length difference wd(xcex) according to             w      d        ⁡          (      λ      )        =            -              w        0              +                  ∑                  k          =          0                          M          -          1                    ⁢                        {                                    a              k0                        +                                          a                k1                            ⁡                              (                                  λ                  -                                      λ                    0                                                  )                                      +                                                                                a                    k2                                    ⁡                                      (                                          λ                      -                                              λ                        0                                                              )                                                  2                            ⁢                              xe2x80x83                            ⁢              …                                ⁢                      xe2x80x83                    }                ⁢                  d          k                    
in which ak0, ak2, . . . =series expansion coefficients of the modified refractive index ak, according to ak=ak0+ak1(xcexxe2x88x92xcex0)+ak2(xcexxe2x88x92xcex0)2 . . . , and in which xcex is a wavelength of the optical beam and xcex0 is a central wavelength of a predetermined spectral band.
In this alternative embodiment, an M-th order fit is obtained for the wavelength dependent phase shift around the central wavelength xcex0.
By changing the design parameters of the first and second refractive means, the phase shift obtained is independent of the wavelength of the optical beam. This allows introducing a constant, wavelength independent phase shift in optical beams having a relatively broad bandwidth.
With the achromatic phase shift device according to the present invention, applications can be made for adjustment, modulation and/or closed loop control of the phase difference of broad band, interfering optical beams.
A second aspect of the present invention relates to an interferometer having a first input plane and a second input plane for receiving at least a first and a second optical beam and an interference plane for letting the at least first and second optical beam interfere, a first optical path being formed from the first input plane to the interference plane and a second optical path being formed from the second input plane to the interference plane, comprising optical path delay means for introducing an optical path difference between the first optical path and the second optical path, characterised in that the interferometer further comprises at least one achromatic phase shift device according to the invention, positioned in at least one of the first optical path and the second optical path.
The interferometer according to the present invention allows introducing a phase shift between optical beams with a broad wavelength range.
Preferably, an achromatic phase shift device is positioned in each optical path. By using an achromatic phase shift device in each optical path, symmetry in the two optical paths is maintained as much as possible, which has particular advantages with respect to maintaining the polarisation state of the optical beams.
For use in planet observations, the interferometer preferably comprises main control means for maintaining the phase shift between the at least first and second beam at a predetermined value, preferably xcfx80 radians, the main control means being connected to the optical path delay means and the first control means. With such an interferometer, it is theoretically possible to obtain a suppression factor of 109.